1. Technical Field
The invention is related to scanning beam video projection systems requiring automatic correction of the geometric distortions in the image projected on a screen. More specifically, the invention is related to video laser projection systems.
2. Description of the Related Art
Numerous techniques are known for correcting geometric distortion in video projection systems. These techniques, however, are limited to removal of a known predetermined distortion, or to removal of distortions sensed internally within a projection video cathode ray tube, for example. These techniques are not adapted to measuring or sensing geometric distortions at a projection screen.
One technique for measuring and sensing geometric distortions at the projection screen of a video projection system, and for correcting such distortions, is disclosed in U.S. Pat. No. 4,085,425. This patent discloses a screen projection alignment system in which the periphery of the screen is blank, except for alignment fiducial areas illuminated by the video projector, the fiducial areas being relatively large (on the order of many video picture elements or pixels) so that the beam alignment control accuracy is limited. At least two relatively large photo sensors which are differentially coupled to an analog output circuit are mounted in the fiducial areas. A change in beam alignment causes one of the sensors to receive less photons than the other sensor, so that the differentially coupled analog circuit senses a voltage between the two sensors and generates an output signal from which may be derived a beam alignment correction to be performed by the video projector. This technique is, therefore, not readily adapted to a scanning beam type of projector, since the two sensors in each alignment fiducial area must be constantly illuminated, rather than being scanned by a small beam. In fact, the referenced patent teaches a light valve type of TV projector, which illuminates all the screen simultaneously, as opposed to a raster type of projector.
3. Problem
In a beam scanning type of video projector, prior known techniques for removing geometric distortion are not suitable, since such techniques either assume a predetemined distortion to be corrected, or required continuous illumination of large alignment fiducial area sensors. Thus, there is a need for an efficient geometric distortion correction system suitable for beam scanning type video projectors--such as video laser projectors--which can take advantage of the small beam diameter for more accurate beam alignment control.